With improved environmental protection awareness of people, an environmentally friendly and energy-saving electric vehicle is playing a role as a replacement of an oil-fueled vehicle, and therefore, electric vehicle technologies rapidly develop. As the electric vehicle technologies develop, people impose a stricter requirement on working performance of a drive system in an electric vehicle. For example, the drive system needs to have a capability of a wide speed adjustment range. That is, the drive system needs to have a relatively high maximum speed.
Currently, referring to FIG. 1, a drive system used by an electric vehicle usually includes a power battery 110, a drive 120, and a motor 130. The drive 120 is connected to both the power battery 110 and the motor 130. The power battery 110 is configured to provide direct current energy, and the drive 120 is configured to: convert a direct current of the power battery 110 into an alternating current, and output the alternating current to the motor 130, so that the motor 130 outputs torque to drive the electric vehicle to travel. The drive 120 includes a bus capacitor C and a conversion module. The bus capacitor C is connected between a positive electrode and a negative electrode of the power battery 110, and is connected to the conversion module in parallel. The conversion module includes six switching transistors V1 to V6. The switching transistors V1 and V4 are connected in series, the switching transistors V3 and V6 are connected in series, and the switching transistors V5 and V2 are connected in series. The serially-connected switching transistors V1 and V4, the serially-connected switching transistors V3 and V6, and the serially-connected switching transistors V5 and V2 are separately connected between the positive electrode and the negative electrode of the power battery. In addition, a node A is extended between the serially-connected switching transistors V1 and V4, a node B is extended between the serially-connected switching transistors V3 and V6, a node C is extended between the serially-connected switching transistors V5 and V2, and the nodes A, B, and C are separately connected to a stator end of the motor 130, so as to output a three-phase alternating current to the motor 130. When the motor 130 runs at a high rotation speed, an end voltage of the motor 130 boosts with the rotation speed. When the end voltage of the motor 130 reaches a maximum voltage that can be output on a direct current side of the drive 120, the conversion module included in the drive 120 may be used to control, by means of field weakening control, the motor 130 to run at a higher rotation speed, that is, the conversion module included in the drive 120 may be used to weaken a magnetic field of the motor 130, so that a rotation speed of the motor 130 continues to boost. In this way, a wide rotation speed range of the electric vehicle is implemented.
When the motor runs at a high speed, in a process of performing field weakening control on the motor, if the magnetic field of the motor needs to be weakened, a direct-axis demagnetizing current component of the motor needs to be increased and a quadrature-axis demagnetizing current component of the motor needs to be reduced. That is, a reactive current component needs to be increased and an active current component needs to be reduced. Because the reactive current component of the motor is increased, a power factor of the motor is reduced. Consequently, output power of the drive system is reduced, and efficiency is reduced.